Many data flow systems have individual processing units (operators) that keep their state in volatile memory, which can lose information upon failure or planned shut-down. The operators may be any of various different kinds of operators that can pass data items between each other within a system (e.g., programs, program components, virtual machines, physical machines, etc.). Data items are units of data that are to be processed by operators, but the data items may be in any of various different forms. For example, the data items discussed herein may be events. One or more input modules or components may provide input to the data flow system, and one or more output modules or components (sinks) may receive output from the data flow system. Some data flow systems process continuous queries. With such queries, the state may represent results of data items that have been processed over a long period of time.